The present invention relates to a bath reboiler-condenser of the type consisting of brazed plates.
The invention applies in particular to the main reboiler-condensers of air distillation plants, which vaporize low-pressure liquid oxygen (typically at a pressure slightly greater than atmospheric pressure) by the condensation of medium-pressure nitrogen (typically at 5 to 6 bar absolute), and it will be explained below in this application.
Bath reboiler-condensers operate as a thermosiphon. The upflow of the vaporizing oxygen is provided by the hydrostatic pressure due to the head of the liquid oxygen bath.
For safety reasons, the recirculating liquid oxygen flow rate must be several times greater than the vaporized oxygen flow rate. For this reason, the head of the liquid oxygen bath must be approximately equal to the height of the exchanger, that is to say the latter is almost completely immersed in the liquid.
If it is desired to reduce the temperature difference between the fluid which condenses and that which vaporizes, so as to reduce the pressure of the heating nitrogen and therefore the energy required to compress the treated air, the heat-exchange area must be increased. Since the horizontal dimensions of the exchanger are limited by the space available in the bottom of the low-pressure distillation column, the height of the exchanger must be increased.
However, such an increase in the height increases the hydrostatic pressure of the liquid at the bottom, that is to say at the inlet of the vaporization passages, thereby creating, in the lower part of the exchanger, a region containing subcooled free liquid, and at least partially neutralizes the beneficial effect of increasing the exchange area.
To avoid this drawback, it has been proposed (see for example U.S. Pat. No. 4,606,745) to subdivide the exchanger into several superposed exchangers, each of which is immersed in a shallow bath. However, this solution has not proved to be satisfactory since a shallow bath results in a low velocity of the vaporized gas leaving the exchanger. This results in insufficient two-phase fluid flow and poorer heat exchange.
The object of the invention is to provide a bath reboiler-condenser with improved thermal performance.
For this purpose, the bath reboiler-condenser, of the type consisting of brazed plates, is, according to the invention, characterized in that it comprises:
a lower exchanger part comprising a first brazed stack of parallel plates, of spacer corrugations and of closure bars, which define a series of first vaporization passages and a series of first heating passages;
means for supplying the first vaporization passages with liquid to be vaporized;
at least one upper exchanger part comprising a second brazed stack of parallel plates, of spacer corrugations and of closure bars, which define a series of second vaporization passages and a series of second heating passages;
means for transferring the vapour coming from the top of the first vaporization passages to a lower inlet of the second vaporization passages; and
means for delivering liquid to be vaporized into the stream of vapour which penetrates into each of the second vaporization passages.
The reboiler-condenser according to the invention may include one or more of the following characteristics, taken in isolation or in any of their technically possible combinations:
the reboiler-condenser comprises means for accelerating the stream of vapour which penetrates each of the second vaporization passages, especially a restriction in the flow area for this stream;
the first and second vaporization passages are bounded respectively by two separate exchanger bodies and the said transfer means comprise a collector for all of the vapour coming from the top of the first vaporization passages, this collector being connected overall to the lower inlets of all the second vaporization passages;
the first and second vaporization passages are bounded by the same plates, the exchanger parts forming a single exchanger body in which the spacer corrugations of each vaporization passage are interrupted in order to receive the said liquid delivery means;
the reboiler condenser includes a feed for feeding liquid to be vaporized only to the said delivery means, the said supply means receiving the excess liquid coming from the upper exchanger part;
the said liquid feed runs into a liquid reservoir open at the top, suitable for creating a hydrostatic head above the said delivery means, this reservoir also receiving the excess liquid coming from the top of the upper exchanger part and running into a space which forms part of the said supply means;
the said supply means are suitable for receiving the excess liquid coming from the top of the first vaporization passages; and
the said liquid delivery means comprise perforated tubes placed at the inlet of each of the said second vaporization passages and, in these passages, the perforations of these tubes being oriented upwards.
The subject of the invention is also an air distillation plant comprising a reboiler-condenser for vaporizing low-pressure liquid oxygen by the condensation of medium-pressure nitrogen, this reboiler-condenser being as defined above.